Over the last century numerous thin film deposition methods have been developed and commercialized. Thin films usually are considered to be less than 10 microns thick. The Handbook of Thin-Film Deposition Processes and Techniques (Noyes Pubs. 1988; Schuegraf, K. K. editor) provides a broad review of thin-film deposition techniques. Of these technologies, chemical vapor deposition, spray deposition and thermal spraying deposition are the most related deposition techniques.
Chemical Vapor Deposition (CVD) is a materials synthesis process in which constituents of the vapor phase react chemically near or on a substrate surface to form a solid product. In most cases, gas phases flow into a reaction chamber where CVD occurs. The reaction occurs at an elevated temperature which is provided by a furnace or by a method, usually RF induction or high-intensity radiation lamps, to heat the material substrate that is to be coated. Plasma, microwave, photo, laser, RF, and electron-enhanced CVD processes have all been developed. Since a reaction chamber and secondary heat source are mandatory with these processes, they are quite different from the combustion CVD (CCVD) of the present invention, which can be conducted in open atmosphere without the need for a secondary heat source.
In the 1940's, a CVD process utilizing a flame to produce homogeneously nucleated (powder) oxides of titanium, zirconium, iron, aluminum, and silicon was developed as disclosed in Swiss Patent No. 265192. By injecting a metal halide vapor and oxygen mixture through the central nozzle of a burner, fuel gas through an intermediate ring, and oxygen through the outer ring, a flame was produced from the burner. The 950.degree. C. to 1000.degree. C. temperature of the flame caused oxidation of the metal halide vapor, which condensed to form very fine oxide powders.
U.S. Pat. Nos. 2,239,551, 2,272,342, and 2,326,059 were granted for producing glass and glass coatings in a flame using combustible gases and the vapor of a hydrolyzable compound of silicon solely or with other possible volatile compounds providing one or more additional oxides such as titania or alumina. These methods use only gas and/or vapor source materials to produce glass coatings, whereas the present method uses a liquid solution to produce glass coatings.
Diamond (carbon) films have been deposited utilizing the inner flame region (reducing region) of a combustion flame of acetylene and oxygen with the carbon source provided by the decomposition of acetylene. Hirose, Y. et al. The Synthesis of High Quality Diamond in Combustion Flame, 89-12 Proc. Electrochem. Soc. (1989); Zhu, W. et al., Growth and Characterization of Diamond Films on Non-Diamond Substrates for Electronic Applications, IEEE Proceedings, May 1991, pp. 621-46; Murakawa, M. et al., An Experiment in Large Area Diamond Coating Using a Combustion Flame Torch in its Traversing Mode, Surf. and Coatings Tech., pp. 22-9 (Dec. 5, 1990). Two-component oxide powders have been made via a combustion flame in a reactor using all vapor sources mixed with nitrogen in a reactor using all vapor sources mixed with nitrogen in a hydrogen-oxygen flame. Hori, S. et al., Characterization and Processing of CVD Powders for Fabrication of Composite and Compound Ceramics, 155 Mat. Res. Soc. Symp. Proc. pp. 3-12 (1989). Carbon black coatings form on materials held in or beyond oxygen deficient flames. For diamond and other pure carbon films, the carbon is deposited from the fuel itself; however, with the present method, a reagent from which the coating is formed is introduced in addition to the fuel.
CVD has been accomplished using a sprayed or atomized solution. Groth, R., 14 Phys. Stat. Sol., p. 69 (1966). One such process, the Pyrosol.RTM. process, involves the deposition from a vapor produced from an aerosol generated by ultrasonically nebulizing a solution of organic or inorganic compounds into a furnace. Blandenet, G. et al., Indium Oxide Deposition on Glass by Aerosol Pyrolysis, 5th Int'l. Conf. on CVD, p. 190-203 (1975). Another method, Pyrolytic Spray.TM., produces aluminum coatings by atomizing warmed aluminum alkyl, as either a pure liquid or as a kerosene dilution, over a heated substrate in a reaction chamber. Withers, J. C. et al., Aluminum Coatings by a Pyrolytic Spray CVD Process, Second. Int'l Conf. on CVD, p. 393-402 (1970). These reactions are confined to a reaction chamber or furnace and call for an external heat source. The basic concept is that the atomized liquid vaporizes prior to reaching the substrate, and then reacts on or near the substrate as in conventional CVD. None utilize a flame or combustion, as does the present invention.
CVD has been accomplished by directly feeding reactive powders such as metalorganics or halides into a furnace. Hollabough, C. M. et al., Chemical Vapor Deposition of ZrC Made by Reactions of ZrC1.sub.4 with CH.sub.4 and with C.sub.3 H.sub.6, 35 Nucl. Tech., p. 527-35 (1977); U.S. Pat. No. 4,202,931; Lackey, W. J. et al., Rapid Chemical Vapor Deposition of Superconducting YBa.sub.2 Cu.sub.3 O.sub.x, 56-12 Appl. Phys. Lett., pp. 1175-7 (1990); U.S. Pat. No. 5,108,983. When viewing this metal organic powder feeding CVD technique through a specially designed end cap on the furnace, the powders flashed or combusted in the hot zone of the furnace. The quality of films (vapor deposited) produced by this process suggests that the powder metal components are vaporized during burning. This is very different from using a liquid solution, and requires use of a furnace, unlike CCVD.
U.S. Pat. No. 3,883,336 discloses a method of producing glass in a flame which appears to be a CVD process even though CVD is not mentioned. A flame from a combustible gas and oxygen mixture is combined with additional oxygen containing silicon tetrachloride vapor which intersects the aerosol of an aqueous salt mixture producing a transparent, homogeneous glass body consisting of at least two constituent oxides. In one of the examples listed, a methanol solution is nebulized and burned by the intersecting flame. The claims only cite the use of aqueous solutions. This procedure is more complicated than CCVD, requiring two nozzles. Further, this procedure is used to make glass bodies and forms, not coatings and films as in CCVD.
Spray pyrolysis is a thin film forming technique in which a solution is sprayed onto a heated substrate, thus forming a film. The film commonly receives additional heat treatment to form the desired phase. One spray deposition method uses metal/2-ethyl hexanoates, spin coated onto a substrate which later is heated to pyrolyze the film and form a YBa.sub.2 Cu.sub.3 O.sub.x (commonly referred to as `123` by those skilled in the art) film. Gross, M. E. et al., Versatile New Metalorganic Process for Preparing Superconducting Thin Films, Appl. Phys. Lett., pp. 160-2 (Jan. 11, 1988). Another method deposits YBa.sub.2 Cu.sub.3 O.sub.x films by spraying nitrate solutions onto substrates at 180.degree. C. followed by heat treating and annealing. Gupta, A. et al., YBa.sub.2 Cu.sub.3 O.sub.x Thin Films Grown by a Simple Spray Deposition Technique, Appl. Phys. Lett., pp. 163-5 (Jan. 11, 1988). A third method, U.S. Pat. No. 5,002,928, forms oxide superconducting films by ultrasonic wave spraying a homogeneous solution or solutions, organic or inorganic, containing as solutes the metal compounds capable of forming the superconductor onto a heated substrate to form the thin film. The substrate temperature can be high enough so that later firing is not needed. These processes do not have a flame and do require heating of the substrate during and/or after deposition, unlike the CCVD of the present invention.
Most thermal spraying methods produce thick films (&gt;10 microns) by feeding powder into a gas combustion torch (flame spraying) apparatus or a plasma torch (plasma spraying) device to melt the powdered coating material, which then is splattered onto the object being coated, thus forming a film. Matejka, D. et al., Plasma Spraying of Metallic and Ceramic Materials (John Wiley & Sons, 1989). Thermal spraying in general is considerably different from CVD and CCVD. An extension of thermal spraying is physical vapor deposition by vaporizing the powdered material, in which the boiling point of the material is exceeded in the thermal sprayer. The resulting vaporized materials then condense on the cooler substrate. This evaporation technique was used early in high temperature superconductor research, and formed c-axis preferred orientation of YBa.sub.2 Cu.sub.3 O.sub.x at deposition rates of up to 10 microns/min. Terashima, K., Preparation of Superconducting Y--Ba--Cu--O Films by a Reactive Plasma Evaporation Method. Appi. Phys. Lett., pp. 1274-6 (Apr. 11, 1988). When only a melting of the YBa.sub.2 Cu.sub.3 O.sub.x was performed, no preferred orientation to the deposited YBa.sub.2 Cu.sub.3 O.sub.x was observed, and the resulting electrical properties were similar to bulk materials. Pawlowski, L. et al., Properties of Plasma Sprayed 123 High Temperature Superconductors, Proc. Third National Thermal Spray Conf., p. 641-6 (May 1990). Most thermal spray processes try to minimize the amount of evaporated powder, because this reduces the efficiency, that is, the ratio of deposited material to in-fed material, of the deposition process. Varacalle, D. J. et at., Plasma Spraying of Zirconia Coatings, 155 Mat. Res. Soc. Syrup. Proc., pp. 235-46 (1989). The films resulting from this process can be very similar to CVD films, but the process is different.
A variation of flame spraying is the feeding of a solution instead of a powder into a flame. Atomized nitrate solutions of Y, Ba and Cu have been reacted in an oxyhydrogen flame to produce fine superconducting powders. Zacharia, M. R. et al., Aerosol Processing of YBaCuO Superconductors in a Flame Reactor, 6 J. Mater. Res., No. 2, pp. 264-9 (February 1991); Merkle, B. D. et at., Superconducting YBa.sub.2 Cu.sub.3 O.sub.x Particulate Produced by Total Consumption Burner Processing, A124 Mat. Sci. Eng., pp. 31-8 (1990). The flame vaporizes the water, leaving the Y, Ba and Cu nitrate particles which react with the flame's OH and O radicals, causing oxidation to yield a YBa.sub.2 Cu.sub.3 O.sub.x particle. A stoichiometric H.sub.2 --O.sub.2 flame temperature is about 2600.degree. C. at atmospheric pressure, but the flame is cooled, by the solution and the addition of Ar, to the 700.degree.-1100.degree. C. range so that vaporization of Y, Cu and Ba is minimized. In this variation, the heat source is not the solution, and the end material is a powder. The best powders were made using a diffusion flame with a flame temperature of 700.degree.-900.degree. C., likely resulting from an over-ventilated diffusion flame, because of the reduced flame temperature in which YBa.sub.2 Cu.sub.3 O.sub.x is a stable phase and the decreased concentration of water vapor minimizing the formation of BaCO.sub.3. The resulting YBa.sub.2 Cu.sub.3 O.sub.x particles deposited on a platinum substrate just beyond the flame typically are in the 20 to 300 nanometer range. The flame also heated the substrate to a temperature of 550.degree.-650.degree. C. The resulting 200 micron thick film from a one hour deposition was particulate in nature with no preferred orientation.
As can be seen, the prior art chemical vapor deposition processes require very specific operating conditions, apparatuses and reactants and carriers. Even with such parameters, many of the prior art processes result in thick films or films with no preferred orientation. Thus, it can be seen that a simple chemical vapor deposition method and apparatus is highly desired but not available.
Furthermore, thin film deposition is useful to impart desired surface characteristics to substrates that would not otherwise have those characteristics. Thin films are useful, for example, to provide corrosion resistance, impart conductivity, provide magnetic shielding, provide debonding in composite materials, and impart desirable optical characteristics to substrates. Accordingly, there is also a need for a method for applying thin films which impart these desirable characteristics to substrates.